Flame spraying involves the heat-softening of the heat-fusible material, such as a metal or ceramic and the propelling of the softened material in particulate form against a surface to be coated to which the heat-fusible material bonds. A flame spray gun is usually used for this purpose and with one type, the heat-fusible material is supplied in powder form to the gun. The powder is of quite small particle size, e.g. below about 100 mesh U.S. Standard screen size, and as small as one micron and is difficult to meter and control.
A flame spray gun normally utilizes a combustion or plasma flame to effect melting of the powder, but other heating means, such as electric arcs, resistance heaters or induction heaters can also be used, alone or in combination. In a powder-type combustion flame spray gun, the carrier gas for the powder can be one of the combustion gases or compressed air. In a plasma flame spray gun the carrier gas is generally the same as the primary plasma gas, although other gases such as hydrocarbon are used in special cases.
To obtain high quality coatings, it is necessary to accurately control the rate of the powder fed through the gun and to maintain the same constant for a given set of spray conditions. The type of fine powder used is a very difficult material to handle and to feed with any uniformity into a carrier gas. While various apparatus of different designs and modes of operation based on gravity, mechanical and gas conveying, and combinations thereof, have been proposed such devices almost universally suffer from a lack of reliability in maintaining a constant controlled powder feed rate and are often subject to mechanical wear and breakdown.
Also, the prior art has found it to be difficult to determine and control the actual amount of powder being conveyed in the carrier gas at any moment of time. While it is possible to accurately measure the rate of flow of the gas stream itself and to measure the amount of powder fed to the stream, it has been difficult to accurately and instantaneously measure the amount of powder actually being conveyed by the carrier gas.
The present invention pertains to and is an improvement over the flame spraying powder feeder described in "Powder Feed Device for Flame Spraying Guns," Ser. No. 605,647, filed Dec. 29, 1966 in the name of Horace S. Daley and now U.S. Pat. No. 3,501,097. In this application, fine flame spray powders are metered into a regulated amount of a carrier gas, the amount of the powder introduced being controlled responsive to the pressure drop in the conveying gas line downstream of the point of powder introduction.
The Daley type of feeder has advanced the art and has ameliorated the problems related to reliability and consistency of powder feed rate. It is mechanical, however, and has had difficulty in handling very fine powders, for example, those with a particle size predominantly smaller than 30 microns. Also, it is large and not suited for mounting directly onto a flame spray gun.